- Ba
- Balancer
- band I
- barbel
- Bars
- Basal body
- basal facet of mouthplate
- Basal feet
- Basal plate
- Basal pores
- Basal ring
- Basal segment
- basale
- basipterygium
- basis
- bathyal
- bathymetric
- bathypelagic
- bayonet chaetae
- Beams
- Bell
- benthic
- benthopelagic
- bicuspate
- Bicuspoid
- bidentate
- bifid
- bifurcated
- Bilamellar wall
- bilateral
- bilaterally symmetrical
- bilobate
- bioluminescence
- biomass
- biotope
- Biramous
- Biramous parapodium
- bisubtropical
- BL
- Blade
- blastostyle
- blastozooid
- Blind canal
- body muscles
- boreal
- brackish
- bract
- Branched spines
- branchial bars
- branchial basket
- branchial cavity
- Branchial heart
- branchial openings
- branchial plate
- branchial sac
- branchial seta
- branchial tube
- branchiostegal spine
- Bristle kinety
- Brosse
- Brosse cleft
- Bubble capsule
- buccal cavity
- Buccal cone
- Buccal connectives
- buccal glands
- Buccal lappet
- Buccal mass
- Buccal membrane
- Buccal membrane supports
- Buccal suckers
- buds
- Bulla
- Bursa seminis
- By-spines